A THICK SOUP OF AEROSOL PARTICLES¿among them sulfates, nitrates, organic and black carbon and fly ash¿fills the skies over northern India in this image.

Last April a dark haze containing more than four times the normal amount of particulate matter in the air slowly drifted eastward over the U.S., sharply reducing visibility in a number of cities. Intriguingly, the haze originated halfway around the world with a massive dust storm in Mongolia. The cloud, roughly the size of Japan, blew across the Pacific Ocean and the continental U.S. before it finally dispersed over the Atlantic Ocean.

Through its travels, this cloud did more than obscure a few views: it dramatically demonstrated the fact that pollution can no longer be considered an urban or regional problem. And because it contained high quantities of aerosols¿microscopic particles such as dust and sea salt, as well as man-made sulfates, nitrates, soot and organic compounds¿it highlighted a need to better understand how these grains affect our planet's weather and climate.

Indeed, although climatologists and other scientists have long focused on gases in the atmosphere, they have not closely examined the role played by so-called condensed phase particles. "Ours is really a younger field in many ways than the study of gas-phase chemistry," says aerosol researcher Barry Huebert of the University of Hawaii. "There was a feeling that if you understood the gas phase, you understood everything that was interesting for a long time." Even when scientists did consider the condensed phase, most thought that because the particles are short-lived, their effects would be temporary. Although the particles might have a regional effect on visibility, they would be rained out of the atmosphere before they had time to really affect climate.

Some crude calculations in the early 1990s (based mainly on sulfates, because the only sufficient data available came from studies of acid rain) showed that aerosols could cool the atmosphere by back-scattering incoming solar radiation. But later studies suggested that aerosols could also warm the atmosphere through their effects on cloud cover and the behavior of less well studied components of pollution, such as soot.

In search of a clearer picture, large teams of scientists from around the world have recently pooled their resources to perform major field experiments involving aircraft, ships, surface stations and balloons. The Asian-Pacific Regional Aerosol Characterization Experiment (ACE-Asia), which took place during last year's dust storms, is the third in a series of experiments designed to investigate different aspects of aerosol behavior in different locales around the world. Another massive undertaking, the Indian Ocean Experiment (INDOEX), meanwhile, was specifically designed to see if climate forcing on the part of aerosol particles could be directly measured.

Conducted in 1999, INDOEX exploited a unique occurrence during the winter monsoon in which air currents carry pollutants from India and Southeast Asia in a southern direction out over the open ocean. South of the equator, however, the air currents are predominantly from the south, keeping the sky relatively free of man-made pollutants. INDOEX planes that flew through the cloud and south over the equator were thus able to sample both the aerosols and a cleaner sky to serve as a reference.

Studying aerosols has not been easy. Because they are short-lived, they do not mix homogeneously around the planet, and so concentrations may differ by many orders of magnitude¿making accurate descriptions of their effects hard to come by. What's more, according to Tim Bates of the National Oceanic and Atmospheric Administration (NOAA), "there's a very wide range of sizes [for aerosol particles], and the effect that the particle is going to have on climate is going to be very dependent on its size, which makes it trickier." Finally, the composition of the particles covers a wide range of chemical species that can combine and interact in numerous ways, all leading to different, and sometimes opposing, radiative effects.

According to a report that used INDOEX data and was published last month in Science, aerosols from man-made pollution may also play a role in weakening the planet's water supply. "We were quite surprised at how massive, how thick this haze layer was," says co-chief scientist Veerabhadran Ramanathan of the Scripps Institution of Oceanography, "and that it cut down on the sunlight going into the ocean by as much as 10 percent." This reduction, in turn, diminished the evaporation of seawater, which feeds precipitation. "So as aerosols cut down sunlight by large amounts," Ramanathan explains, "they may be spinning down the hydrological cycle of the planet."

The study further examined another way in which aerosols can influence the water supply: by exerting control over cloud formation. The presence of these particles helps to seed clouds by providing sites on which water droplets can condense, but very small particles, the scientists found, actually decrease the precipitating efficiency of clouds, meaning it rains less.

Image: UCSD

AEROSOLS influence the planet's water supply by exterting control over cloud formation and reducing rainfall.

"What happens when you have urban and industrial pollution," ACE-Asia scientist Huebert explains, "is that you wind up with so many small particles that you wind up with a very large number of very tiny droplets that are too tiny to settle out [of the cloud]. As a result, you never get that process going where one drop collects another, collects another and it gets bigger and starts going faster and you get precipitation." In the Science paper, Ramanathan and colleagues note that "the suppression of precipitation by aerosols prolongs their atmospheric residence time, further enhancing their impacts."

The researchers also highlighted the need to consider the uneven absorbing effects aerosols can have at different elevations. Radiation-absorbing black carbon, for instance, opposes the cooling effect of sulfates and organics at the top of the atmosphere. At the surface, however, all aerosols reduce the solar radiation. "At present, these effects are not generally accounted for in climate model prediction studies," study co-author J. T. Kiehl of NCAR notes. "But we will need to include the absorbing aerosols in future model predictions."

Such predictions can only benefit from the relative wealth of data concerning aerosols that scientists have recently collected. Although there are no current plans to repeat field experiments for either ACE-Asia or INDOEX, Bates notes that many surface sites and light radar (lidar) stations continue to collect valuable information. And since the researchers are still analyzing data collected over the past few years, a clearer picture of aerosol behavior might yet still emerge from the haze.